The bacterium Bordetella pertussis is the causative agent for whooping cough, a respiratory disease that can be severe in infants and young children. The clinical course of the disease is characterised by paroxysms of rapid coughs followed by inspiratory effort, often associated with a characteristic ‘whooping’ sound. In serious cases, oxygen deprivation can lead to brain damage, however the most common complication is secondary pneumonia.
Whooping cough is usually considered to be caused by B. pertussis, but occasionally B. parapertussis is isolated from patients with typical signs and symptoms of whooping cough. B. parapertussis infection is of lower frequency than B. pertussis with 5-10% of whooping cough being associated with B. parapertussis (Mertsola (1985) Eur J Clin Microbiol 4; 123; Lautrop (1971) Lancet 1(7711) 1195-1198). B. parapertussis is associated with mild clinical symptoms which, combined with its serological cross-reactivity with B.pertussis, makes B. parapertussis difficult to diagnose.
The first generation of vaccines against B. pertussis were whole cell vaccines, composed of whole killed bacteria. These were introduced in many countries in the 1950s and 1960s and were successful at reducing the incidence of whooping cough. A problem with whole cell B. pertussis vaccines is the high level of reactogenicity associated with them. Acellular vaccines containing purified B. pertussis proteins are less reactogenic and have been adopted for the vaccination programmes of many countries. Acellular vaccines typically containing pertussis toxin (PT), filamentous haemagglutinin (FHA) and quite often pertactin (PRN), are widely used and provide effective protection from the severity of whooping cough.
Despite vaccination, whooping cough remains an endemic disease (Mooi et al (2001) Emerging Infectious Diseases 7; 526). Whooping cough has re-emerged in Australia, Canada and The Netherlands; countries with highly vaccinated populations. A comparison of pre-vaccination strains with strains isolated recently, has shown antigenic drift, particularly in PT and PRN (Mooi et al (1998) Infection and Immunity 66; 670). It is widely acknowledged that current vaccines protect against severe disease but do not eliminate Bordetella pertussis from the body (Cherry et al (1998) Vaccine 16; 1901, Hewlett and Halperin (1998) Vaccine 16; 1899, Storsaeter et al (1998) Vaccine 16; 1907). The defence mechanisms of Bordetella pertussis allow it to evade elimination from the body, indicating that current vaccines do not completely disable these defence mechanisms.
Vaccination using whole cell B. pertussis vaccines (Pw), appears to protect against B. parapertussis infection, probably due to the similarity of the two bacteria. B. parapertussis infection in unvaccinated infants may lead to severe and fatal complications, whereas in individuals vaccinated with Pw, a milder, often subclinical course of whooping cough is seen (Long et al (1990) Pediatric Infect Dis J 9; 700). Theoretically, the introduction of acellular pertussis vaccines containing only two or three purified proteins could reduce the ability of vaccination to protect against B. parapertussis. 
Accordingly, further improved acellular vaccines against whooping cough are required that combine low reactogenicity with an ability to elicit a protective response against Bordetella, particularly both B. pertussis and B. parapertussis, infection. The identification of new candidate antigens and particularly effective combinations of antigens will allow the development of such vaccines.